Hello,
I want users to sort posts inside topic by number of likes received. In this forum there are few topics about sorting “topics” in homepage but I did not see anything about “sorting inside topic.”
I am using Discourse as kind of Q&A so I’d like to see this feature 
PS: Do not bother to say that Discourse is not aimed for Q&A 
Without a plugin (that you would need to create, I don’t believe one exists for this) you cannot modify the order of posts in a topic.
Its a very complex change, even with a plugin. As far as I know, no plugin exists that does this.
(also a bunch of features would become super weird in this mode - in reply to in particular)
This is now possible with the Discourse Post Voting plugin 